Polyurethanes are typically prepared by reacting a diisocyanate with a relatively high molecular weight polyol. The diisocyanate can be aromatic, such as tolylene diisocyanate, aliphatic, such as hexamethylene diisocyanate, or cycloaliphatic, such as 4,4'-dicyclohexyl diisocyanate, and the polyol can be of the polyester or polyether type. A diol or diamine containing from 2 to about 10 carbon atoms can be included among the reactants used to prepare the polyurethane as a chain extender to vary the properties of the final polymer. The prior art describes numerous types of polyurethanes and methods for their preparation.
Polyurethanes derived from aliphatic or cycloaliphatic diisocyanates retain their mechanical properties upon exposure to light and oxygen. Polyurethanes derived from aromatic diisocyanates not only lose their tensile properties under these conditions, but also decompose to form highly toxic aromatic amines when exposed to the elevated temperatures required to fabricate these polymers.
A shortcoming of many polyurethanes derived from aliphatic or cycloaliphatic diisocyanates and diol type chain extenders is their tendency to lose 60 percent or more of their initial tensile strength in environments where the relative humidity is above 50 percent. This has prevented the widespread acceptance of this class of polyurethanes in medical applications such as catheter tubing and surgical implants. In these applications the polymer must retain a useful level of tensile properties in the presence of aqueous solutions and in environments where the relative humidity exceeds 50%.
The hardness of a polyurethane is determined at least in part by the ratio of the combination of diisocyanate and chain extender present to polyol and the molecular weight of the polyol. Polyurethanes classified as "soft" typically exhibit durometer values of less than 90, measured on the Shore A scale. The molar ratio of the combination of diisocyanate and chain extender to polyol in these polymers is typically 3:1 or less. Expressed another way, the "hard" segment formed by the combination of diisocyanate and chain extender constitutes less than 80 mole percent of the polyurethane. U.S. Pat. No. 3,923,747, which issued to Kolycheck on Dec. 2, 1975 discloses polyurethanes of this type prepared using aliphatic diisocyanates.
U.S. Pat. No. 3,886,226, which issued to Asai et al. on May 27, 1975 exemplifies polyurethanes wherein the "hard" segments constitute more that 80 mole percent of the polymer, however all of these polymers are derived from aromatic diisocyanates. This limitation is not mentioned in the patent, and the claims are sufficiently broad to encompass polymers containing less than this concentration of hard segments. There is no teaching in the patent regarding the criticality of the aforementioned 80 mole percent limitation with respect to the tensile properties of polyurethanes derived from aliphatic diisocyanates.
Polyurethanes useful as coatings for glass and other substrates are disclosed in U.S. Pat. No. 4,098,742, which issued to Mueller on July 4, 1978. Both aromatic and aliphatic diisocyanates are considered useful starting materials for reaction with a specified class of perfluorinated diols and hydroxyalkyl substituted polyorganosiloxanes. Two of the exemplified polymers are derived from aliphatic diisocyanates and contain more than 80 mole percent of "hard" segments, however no polyester or polyether type polyols are used as reactants, nor is there any indication that any of the polymers disclosed in this patent are useful for anything other than coating materials.
European Patent Application No. 68,385, published on Jan. 5, 1983, discloses linear block copolymers containing polyurethane and polydiorganosiloxane segments. The polydiorganosiloxane segments constitute from 1 to 50 weight percent of the polymer, and form at least a portion of the "soft" segments of the polymer. The "hard" segments include an aromatic or aliphatic diisocyanate and any chain extenders. The chain extenders are typically low molecular weight diols or diamines. A characterizing feature of these polymers is the ratio of the total molecular weight of the soft segments to the total molecular weight of the polymer, which is from 0.4 to 0.8, however there are no limits defined for the relative concentrations of hard and soft segments other than an upper limit of 50 weight percent for the silicone portion, or the effect of this variable on the physical properties of the polymer.
European Patent Application No. 78,958, published on May 18, 1983, discloses polyurethanes derived from aliphatic, cycloaliphatic or aromatic diisocyanates, high molecular weight organic compounds containing two hydroxyl groups, a difunctional chain extender and from 1 to 15 weight percent, based on the final polymer, of a polydiorganosiloxane having isocyanate-reactive terminal groups. The molar ratio of isocyanate groups to reactive groups of the other ingredients used to prepare the final polymer is from 0.85:1 to 2:1, and the molar ratio of diisocyanate to chain extender can be from 0.2:1 to 30:1. Expressed another way, the "hard" segment consisting of isocyanate and chain extender units can constitute as little as 50 mole percent of the total segments present in the final polymer.
The prior art of which we are aware is silent regarding the adverse affect of humidity on the tensile properties of polyurethane elastomers derived from aliphatic or cycloaliphatic diisocyanates, or the desirability of incorporating at least 80 mole percent of "hard" segments into these polymers.